A strain gauge is a device used to measure mechanical deformation of an object. The most common type of strain gauge consists of a flexible backing which supports a metallic foil pattern. As the object is deformed, the foil pattern is deformed, causing its electrical resistance to change and this resistance change is correlated with the extent of deformation. The active strain element does not have to be in foil form but can also be a wire or a (metallic) thin film. Although the present application describes the disclosure mostly in reference to a strain gauge, the principles discussed herein are equally applicable to sensing other properties related to deformation, including shear, torsion, and other static properties.
Strain gauges are used for measuring strain in a surface. One of the most common uses of strain gauge technology is to verify if an object or a component has deformed to the same extent as predicted under a preset load or stress level. Strain gauges are useful for experimental stress analysis and the experimental results are usually compared with those results established from finite element analysis software and other simulation tools such as ANSYS or ABAQUS.
Current strain gauge technologies include electrical resistance strain gauges (which employ wires, foils or thin films), capacitance strain gauges, fiber optic or optical strain gauges and variations of these. Resistance strain gauges applied to engine components such as blades and vanes are usually accessed using slip ring technology or telemetry systems and the signals are brought to this hardware using wires encased in an Inconel sheath.
Optical strain gauges can exhibit significantly greater sensitivity than traditional metal foil strain gauges, are light weight and easily manufactured, and are easily incorporated into optical fiber data collection/distribution systems. Fiber optic strain sensors are a popular way to monitor structures, since several very long optical fibers can be used to gather distributed strain and temperature data from many points over very large structures. Optical strain sensors can be fragile, and require a physical connection to measure data.
A large amount of prior art exists describing various types of strain gauges including resistance-based using flexible backing with metallic or wire. Typically, foil or wire strain gauges have figures of merit or gauge factors (G) on the order of 2-4. Semiconductor gauges have gauge factors that are typically 1-2 orders of magnitude greater than metal foil gauges. A measure of the responsiveness or strain sensitivity of resistance strain sensors is given by the gauge factor (G), which is defined according to the equation below
  G  =                    Δ        ⁢                                  ⁢        R            R        *          1      ɛ      where R is the electrical resistance, ΔR is the change in resistance and ∈ is the applied microstrain.
Strain gauges must be properly attached to the surface to be tested to accurately measure strain. In the gas turbine engine environment, adhesion is a major issue since the gas velocities can approach 1000 ft/sec and the “g” forces acting on the strain gauges attached to blades or discs are extreme under typical rotating conditions. Attachment using adhesives, etc. is therefore problematic, making thin film sensors very attractive from this viewpoint. Thin film strain sensors are well suited to make direct physical measurements on turbine components in harsh environments and thus, assess the mechanical behavior of these components during actual engine operation, so that structural models can be validated and newly developed materials evaluated. Thin film sensors are non-intrusive in that the gauge thickness is considerably less than the gas phase boundary layer thickness and thus, the gas flow path through the engine will not be adversely affected by these sensors. Thin film sensors have negligible mass and thus are not affected by the large “g” forces acting on the blades.
Current strain gauge technologies include electrical resistance strain gauges (which employ wires, foils or thin films), capacitance strain gauges, fiber optic or optical strain gauges and variations of these. Resistance strain gauges applied to engine components such as blades and vanes are usually accessed using slip ring technology or telemetry systems and the signals are brought to this hardware using wires encased in an Inconel sheath. Wire strain gauges employed in the hot sections of gas turbine engines are usually imbedded in a ceramic coating. Typically these gauges are imbedded in a sprayed ceramic coating (thermal spray instrumentation) which is applied to a MCoCrAlY bond coat for adhesion purposes. The wires are affixed to the sprayed ceramic surface coating and an additional layer of ceramic is deposited over the wire instrumentation in effect forming an imbedded sensor element. The foil gauges are usually attached with a backing or an adhesive to the substrate that is to be evaluated. This cannot be done for higher temperature applications due to adhesive issues, etc. and this approach is marginal at best even in the colder sections of engine. Since today's compressor blades are becoming thinner and thinner, the mass of the attached sensor itself (and associated bonding agents) can compromise the strain measurement by altering the various vibrational modes; i.e. their low mass will not affect the blade's balance. Since thin film strain gauges are deposited directly onto the substrate of interest without any adhesive, they are attractive from many viewpoints including adhesion, mass and durability.
Capacitance-based strain gauges operate in a similar manner to resistive strain gauges in that a change in strain is translated into a change in capacitance (vs. a change in resistance). The change in capacitance must be measured (generally in a powered circuit) and communicated to an external device or system for use (generally using a wired connection).
Current optical strain gauges can be classified as either intensity based (OSG) or interference based (FBG). The Fiber Bragg grating (FBG) strain sensors employ a grating in the solid fiber and a shift in wavelength of the reflected light off the grating is correlated with the amount of strain in the fiber. The intensity based OSG relies on a hollow capillary glass-tube with specialized thin film coatings on the outside surface to attenuate and reflect the light launched into the tube from an optical fiber source. A second fiber, at the other end of the tube, carries the attenuated light beam to a detector. The amount of attenuation is based on the bending of the tube, which affects both the angle of incidence and the number of bounces that various light rays experience while inside such a hollow wave-guide. The bending radius is, in turn directly related to the strain experienced by the tube and is transferred to the tube by the structural component to which it is affixed.
Suitable modifications of the surface coatings permit tailoring the gauge factor (G) to specific strain ranges. Hollow tube strain gauges have demonstrated gauge factors in the range from 100 to 300. Metal strain gauges, in contrast, have gauge factors in the range 2 to 4 and require some method of compensation if the ambient temperature changes. For the strain gauges of the intensity type described here, G˜120, bending strains to 3000 μE (micro-strain) were readily monitored over a temperature range from −20° F. to 115° F. without requiring temperature compensation. As with a metallic strain gauges, a gauge factor can be defined as the change in intensity divided by the initial intensity per unit of strain.
  G  =            1              I        0              ·                  Δ        ⁢                                  ⁢        I                    Δ        ⁢                                  ⁢        ɛ            
While G thus defined is not constant for all strains, a range can be determined for each OSG, controlled by the absorptive layer, for which a linear approximation is valid. Indeed this is an advantage of the OSG, since the gauge factor can be tailored for a specific strain range. Optical strain sensors can be fragile, and require a physical connection to measure data.
There is limited existing art for wireless strain gauges. In one known configuration, the measurements from a strain gauge are transferred from the gauge via a wired connection or an fiber optic cable connection, and then connected to a wireless transmitter (such as an 802.11 node). This embodiment is not wireless under the definition used in this disclosure as the device must have an intermediate direct wired or fiber connection. As used in this disclosure a wireless strain gauge has not physical connection with the communication path that transfer the measurement from the gauge to the instrumentation that reads and interprets the measurement.
Another prior art system is described in U.S. Pat. No. 6,622,567 which refers to a system that has a strain gauge made up of a differentially variable reluctance transducer (resistance-based strain measurement) coupled with an RFID device. It describes an external reader that sends electromagnetic energy to the strain gauge/RFID that is used to temporarily provide power so that the gauge can make a strain measurement, and the RFID device and strain gauge can send signals back to the reader. This embodiment is different than the present disclosure in several ways. First, the strain measurement is made via a Wheatstone Bridge-based resistive measurement. Second, the patent describes an RFID device as an integral part of the invention. Third, the gauge/RFID device has active circuit elements that are powered by the reader-supplied electromagnetic energy. In contrast, the present disclosure utilizes a frequency shift induced single capacitive measurement, utilizes an interrogating RF signal which is transponded with encoded strain information, and all circuit elements in the strain gauge and associated microwave communications circuit are passive, and the device is never powered.
Another piece of relevant prior art is the work being done under the NASA/DoD UAPT program. This prior art describes approaches to “remote sense parameters in harsh environment using self packaged, passive wireless sensors.” However, there are several significant differences between this prior art and the present disclosure including this prior art does not address strain gauges, uses different materials as the basis for the sensors than the materials disclosed herein, does not disclose a means for the implementing the transmitter/receiver/signal processor portion of the system and specifically does not provide a means for detecting and measuring the response signals in presence of the interrogation signals. This prior art is directed to magnetic loop-based antennas rather than electrical dipoles as described in the current disclosure.
Although remote sensing requires sensors that consume no or very low power for reading the sensor and transmitting the data, most prior art sensors require continuous energizing either for operation or for data transmission, and have required substantial power supplies. For example, a paper, “Multichannel Strain Gauge Telemetry for Orthopaedic Implants,” by G. Bergmann, et al., J. Biomechanics Vol. 21 no. 2 pp 169-176, 1988, describes remote powering of a Wheatstone bridge with active strain gauges that require continuous power. A paper, “Remotely powered, multichannel, microprocessor based telemetry systems for smart implantable devices and smart structures,” by Christopher Townsend, et al, described an implantable sensor telemetry system that uses low power microprocessors integrated circuits, Wheatstone bridge signal conditioning, and a remote powering system. The Wheatstone bridge has advantage in providing temperature compensation. However, the bridge circuit also requires a continuous voltage and flow of current, so substantial energy is eventually used. “Capacitive Sensors Design and Applications,” by L. K. Baxter, IEEE Press, 1997, shows a microcontroller providing a train of pulses or a microcontroller providing a single interrogation pulse to excite a capacitive limit switch. However, the circuit described by Baxter does not provide a way to measure more than the two positions of the capacitor and does not compensate for changes in temperature.
Fiber optic strain sensors are a popular way to monitor structures, since several very long optical fibers can be used to gather distributed strain and temperature data from many points over very large structures. However, these sensors require expensive equipment for monitoring and acquiring data. They also require special care during installation to prevent fiber breakage during the construction process. Fiber optic connectors, which allow the embedded fibers to egress from the structure in order to be interrogated by external equipment, adds to construction costs and to the difficulty of sensor installation. Furthermore, like previously described systems, they also require power, which can fail at critical moments, such as during a violent storm when important data needs to be collected.
Conventional bonded foil strain gauges are subject to drift due to delamination of their bond to the structure under test over time, and with exposure to moisture. Considerable surface preparation of the structure, and exotic coating steps are required in order to maintain reasonable short-term recordings from a bonded gauge to be exposed to the environment. More stable vibrating wire strain gauges are typically welded to the structure under test, but these welds may cause localized corrosion It is important to note that neither bonded nor vibrating wire strain gauges in the existing art include a way to passively detect peak displacements or strains.
The prior art gauges discussed above are the general purpose type used in most operating environments. However, these strain gauges may not be suitable in harsh environments such as with the turbine blades. Currently, wire connected indium-tin-oxide (ITO) based resistive sensors are used for strain measurements in turbine engines. As an object is deformed, the active strain element usually a resistor element is deformed, causing its electrical resistance to change and this resistance change is correlated with the extent of deformation. In the ITO sensors the electrical resistance of the ITO changes with strain due to the Poisson contraction (geometrical effect) as well as a strain induced resistivity effect (common to semiconductor strain elements). The change in electrical resistance of the active strain element is monitored using wires that are eventually connected to a slip ring or telemetry system and then to the outside world. These electrical devices require wires to get the signal from the engine environment to the data acquisition system for data collection and analysis. The ITO sensors have much larger gauge factors and thus don't suffer from the low signal to noise ratios that plague slip ring technology. The piezoresistive response of semiconductor gauges similar to the ITO gauge described here, depends largely on the changes in band structure as the semiconductor is deformed. This is in contrast to the metal/alloy based strain sensors where changes in dimension of the active sensor element due to straining are responsible for changes in electrical resistance. Thus, the piezoresistive response of ITO based strain sensors is 1-2 orders of magnitude greater than those observed in metals. However, all of these sensors require lead wires to take the signal from the sensor to the outside world and this makes the measurement expensive and cumbersome, even with the larger gauge factors possible with the ITO based strain sensors. Therefore, techniques such as passive wireless strain gauge that can eliminate the use of slip rings are attractive from the viewpoint of cost and complexity of the measurement. The implication here is that potentially every blade could be instrumented and the possibility of engine health monitoring would then be feasible which brings a whole new dimension to aviation maintenance and safety. These are the main advantages of the gauge disclosed here as compared to the existing resistance strain gauges being used by the industry today.
One of the merits of the thin film strain gauges being developed today for the turbine section of engines is their high temperature capability. Thin film strain gauges based are being developed to measure static and dynamic strain at temperatures approaching 1500° C. These ceramic strain gauges exhibit excellent oxidation resistance and high temperature stability, surviving tens of hours of strain testing at temperatures as high as 1530° C. in air.
Ceramic strain gauges based on reactively sputtered indium tin oxide thin films have been developed over the past ten years to monitor the structural integrity of components employed in propulsion systems that operate at temperatures in excess of 1500° C. This has been largely accomplished using microstructural control during the fabrication of the thin film ceramic sensors and is the subject of a recent patent. Here the microstructure of the ITO strain sensors consisted of a contiguous network of indium tin oxide (ITO) particles with well-defined necks between the individual ITO particles. These ITO films were prepared by sputtering in various nitrogen/oxygen/argon partial pressures to incorporate large amounts of nitrogen into the films. Under these conditions, sintering and densification of the ITO particles containing these nitrogen rich grain boundaries were retarded and a contiguous network of nano-sized ITO particles was established. The enhanced electrical conduction along the surfaces of these contiguous ITO particles resulted in a very stable and large piezoresistive response with a gauge factor of 22 and a drift rate of 0.0001%/hour at temperatures as high as 1575° C. In addition to the stabilization of the ITO strain sensors to very high temperatures, novel temperature compensation techniques have been implemented over an extended temperature range. This includes the placing of noble metal resistors in series with the ITO strain sensors to form a self-compensated ceramic strain gauge that exhibit extremely low TCR's from room temperature to 1400 C (U.S. Pat. No. 6,729,187). Other techniques are also being developed to reduce the TCR of the ITO gauges even further, which is the subject matter of another recent patent. All of these technologies are important to improving the signal to noise ratio of these resistance strain gauges, which must ultimately rely on slip ring technology to get the signal from the sensor to the data acquisition system.
Prior art optical strain gauges are generally not suitable for operating at high temperatures since the diffraction grating (Bragg grating) must be stable (no diffusion effects). The current silica based glass fibers will not work at high temperatures and alternative solid fibers must be utilized. Therefore, some high temperature work has been attempted using sapphire fibers as the active elements in the FBG optical strain gauges. Intensity based optical sensors (OSG) have been developed for low temperature applications but are still under development for high temperature applications as well. The sensors are based on a hollow capillary glass-tube with specialized refractory thin film coatings on the outside surface to attenuate and reflect the light launched into the tube from an optical fiber source. A second fiber, at the other end of the tube, carries the attenuated light beam to a detector.
The amount of attenuation is based on the bending of the tube, which affects both the angle of incidence and the number of bounces that various light rays experience while inside such a hollow wave-guide (U.S. Pat. No. 6,850,315).
None of the current strain gauge technologies are ideally suited to measure strain in turbine engines, especially under normal operating conditions. All current technologies exhibit some attribute shortcoming that prevents effective and efficient strain measurement.
This disclosure describes a strain gauge and accompanying instrumentation system that overcomes prior art limitations. This disclosure describes a passive microwave circuit that both measures strain and converts the measurement to an RF signal that can be accessed and used outside of the turbine engine without a physical connection. The thin, low mass microwave circuit is deposited or attached directly on the engine fan blade without materially affecting the blades balance or the engine gas flow and can operate in the harsh physical environment internal to a turbine propulsion engine. A novel transponder circuit and external transmitter/receiver/processor provides the interrogation, response and signal processing to estimate the mechanical strain from the RF signal.